Laser quenching technology, also known as laser heat treatment or laser transformation hardening process, employs a laser beam to irradiate a metal workpiece, enabling surface temperature thereof to rise above austenitizing temperature Ta. After removal of the laser beam, temperature of the laser treated region drops below the temperature of martensitic transformation quickly, and a martensitic hardened layer is formed in the surface area because the cooling rate of the heated region is greater than the critical cooling rate of quenching due to rapid heat conduction of the base material which is still in the room temperature for it is not heated directly. Laser quenching belongs to a self-cooling quenching process for its rapid cooling rate and absence of cooling mediums such as water and oil.
Generally, the laser quenching process is classified into two categories: one is known as laser transformation hardening or laser heat treatment process, in which a metal surface does not melt and only a solid-state transformation occurs after laser irradiation, and its primary feature is to ensure that the maximum temperature of the metal surface is below its melting temperature Tm during laser irradiation, therefore, process parameters of the laser quenching (including laser power, spot size, scanning speed, etc.) should be properly selected. The other is known as melting laser quenching process, in which the temperature of a metal surface may now exceed its melting point and the surface may be melted after laser irradiation. Since a surface of a workpiece is melted, a higher laser power and a slower scanning speed can be used, besides, the hardened layer is deeper than that of a typical laser quenching process. However, the melting laser quenching process significantly alters surface roughness of the metal material, so use thereof is limited in circumstances where a high precision is required and a subsequent machining is forbidden. Sometimes, a local micro-melting may occur in the surface of a metal workpiece due to improper selection and fluctuation of process parameters, and the micro-melting layer can be removed by polishing or grinding. The process is generally still attributed to the laser quenching process. Unless specified otherwise, the laser quenching described hereinafter refers to the process of solid-state transformation hardening in which a metal material hardly melts or only local micro-melting occurs.
A depth of a hardened layer by laser quenching is not only related to process parameters such as laser power, scanning speed and spot size, but also related to the thermal conductivity and the hardenability of a metal. For a specific metal material, its austenitizing temperature Ta and melting temperature Tm are approximately stable and only vary with fluctuations of microstructures and the uniformity of the overall composition. Generally, the conduction depth of a metal workpiece with a temperature higher than the austenitizing temperature Ta determined by laser process parameters and the procedure of heat conduction corresponds to the depth of a hardened layer by laser quenching.
The depth of a hardened layer by laser quenching is not only related to parameters of laser quenching process, but also related to the thermal conduction process of the base of a metal material, and particularly closely related to the thermal conductivity of the material, which is jointly determined by parameters of laser quenching process and the thermal conduction properties of the base. During laser quenching, laser output modes include continuous output and pulsed output scanning quenching. The thermal conduction process of a existing scanning laser quenching, either continuous or pulsed laser quenching, can be analyzed by the thermal conduction equation of a continuously fixed point-like heat source, and the equation of the thermal conduction temperature is as follows:
                              T          ⁡                      (                          R              ,              t                        )                          =                              p                          2              ⁢              π              ⁢                                                          ⁢              λ              ⁢                                                          ⁢              R                                ⁡                      [                          1              -                              ϕ                ⁡                                  (                                      R                                                                  4                        ⁢                        a                        ⁢                                                                                                  ⁢                        t                                                                              )                                                      ]                                              (        1        )            
In formula (1), R is the distance from a point to a heat source; T(R,t) is the temperature of a point in the surface of a workpiece at a distance R from the heat source at a time t; p is the effective power of the heat source; t is the thermal conduction time in the metal; λ is the thermal conductivity of the metal; a is the thermal diffusivity of the metal; and ϕ(u) is a probability integral function. When t=∞, the heating time of the heat source can be considered to be infinite, thus ϕ(u)=0, and the ultimate supersaturation temperature Tsp of a point at a distance R from the laser point source is as follows:
                              T          sp                =                              p                          2              ⁢              π              ⁢                                                          ⁢              λ              ⁢                                                          ⁢              R                                ⁢                                          ⁢          or                                    (        2        )                                R        =                  p                      2            ⁢            π            ⁢                                                  ⁢            λ            ⁢                                                  ⁢                          T              sp                                                          (        3        )            
Where Tsp is proportional to the inputted laser energy, and is inversely proportional to the distance R from the heat source. For laser quenching process, it is obvious that Tsp should not exceed the melting point of a metal material. Since it is necessary that the temperature of a heated region should exceed the austenitizing temperature to form a laser hardened layer, Tsp>Ta. Therefore, a prerequisite for laser quenching to obtain a martensite is that the range of the temperature of a laser heated region Tsp is as follows: Tm>Tsp>Ta.
According to the equation of heat conduction (1) and the equation of heat conduction (2) or (3) under the condition of ultimate supersaturation, the following conclusions can be derived:
(1) The longer the time of laser heating, or the higher the injected energy density, or the greater the laser beam absorptivity of a metal material, or the greater the thermal diffusivity of a metal material, the higher the temperature T(R, t) of the metal will be, the deeper the part beneath the surface capable of reaching the austenitizing temperature will be, and correspondingly the greater the depth (R) of the laser hardened layer will be.
(2) After the material for quenching is determined, the depth (R) of the laser hardened layer is closely related to laser power (p), spot size, power density and treating duration.
The process of laser quenching in the prior art always adopts a focused spot for scanning quenching. There are two shapes of laser spots: one is a circular spot; and the other is a rectangular spot obtained by optical shaping. As surface melting is not allowed in laser quenching, neither excessive laser power or laser power density nor excessive treating duration is to be adopted, therefore, the depth of a hardened layer by laser quenching processes in the prior art is extremely limited according to the three equations described above.
In recent years, selective laser quenching process has been used more and more widely. Unlike conventional laser quenching processes hardening the entire surface of a metal workpiece, selective laser quenching process selectively hardens parts of the surface of a material by a laser beam in terms of the requirements of the workpiece properties, namely, the hardened regions do not cover the entire surface of the workpiece, and form a compound soft-and-hard hardened layer or hardened arrays. In this way, better wear resistance and better and toughness of a metal surface can be realized. Nowadays, there are many methods to realize the process of selective laser quenching, such as progressive scanning by multi-axis control of the movement of the laser beam or the workpiece, or combining pulsed laser output and the control of the trajectory of the machine tool. Among them, the pulsed laser quenching process can output a pulsed laser by the shutdown function of a switching power supply directly or by a chopper disk changing a continuous laser beam into a pulsed manner. The latter requires a higher accuracy of the control system of the laser quenching machine tool. In addition, selective laser quenching hardening can also be realized by continuous laser scanning through a mask, then only part of a workpiece can be heated to quench by a laser beam passing through the mask, and parts covered by the mask have no quenching effect. Although the process is simple and does not require a complex control system and programming procedure, it has a relatively low processing efficiency. It must be pointed out that, no matter what kind of method it is, all the existing methods for laser quenching are based on single-scan quenching by a laser beam.
Since no melting in a workpiece surface is allowed in a laser quenching process, and the moving speed of a machine tool is generally low, the laser power and the power density should not be too high, and the quenching speed should also be controlled to a low level if single-scan quenching by a laser beam in prior art is adopted, regardless of continuous laser quenching or pulsed laser quenching. Besides, considering restrictions of thermal conduction properties and the hardenability of the metal material, a laser hardened layer is relatively thin (usually below 1 mm), and the productivity is unable to be improved effectively. With the development of laser devices, the power of solid-state lasers (including fiber lasers) and gas lasers has reached a relatively high level (e.g. the power is 40 kW for fiber lasers and is 20 kW for gas lasers). Those high power lasers can only be used for welding, cutting, cladding, alloying and fusing, in which a material is in a molten state. As for a laser quenching process, both the laser power and the scanning speed should be restricted to a relatively low level to avoid a workpiece melting in the laser quenching process. For example, the typical power of laser quenching is generally 1˜3 kW, and the scanning speed is generally 300˜2000 mm/min. As a result, laser quenching processes in prior art feature in small depth of a hardened layer and low production efficiency, and is difficult to meet the demand for high efficiency in laser production, which hinders further application of laser quenching.
Therefore, it has become one of the key technical problems for further expanding laser quenching's industrial applications that whether a new breed of laser surface quenching can be developed so as to greatly improve the speed and productivity of laser quenching.